Sir Menzies sets tests for Brown
Lib Dem leader Sir Menzies Campbell has paved the way for a potential deal with Labour as he set out five "tests" for a Gordon Brown-led government.
And in an apparent shift, proportional representation for general elections was not included in the list.
There has been speculation the Lib Dems will seek a coalition in the event of a hung parliament.
And a senior party official suggested the sticking point of electoral reform was no longer a top priority.
He said it was "not a deal maker or a deal breaker" in any negotiations to form a government, should no single party hold an overall majority.
"We don't say 'don't pick up the phone unless the single transferable vote is offered for Westminster'," he told reporters.
He said Sir Menzies would take a more "flexible approach" to proportional representation than his predecessors and it was no longer the Lib Dems' top priority when it came to co-operating with other parties.
But in a sign of disagreement within Sir Menzies' inner circle over the party's position, his chief of staff Ed Davey told BBC News 24 he "did not recognise" the source of the story, adding "I briefed the leader's speech and I didn't say that".
'Courage'
Opinion polls point strongly towards a hung parliament at the next election, with the Liberal Democrats potentially holding the balance of power.
In his closing speech, to his party's spring conference in Harrogate, Sir Menzies sent a direct message to Gordon Brown, the man expected to take over from Tony Blair as the next prime minister.
He challenged Mr Brown to have the "courage" to embrace liberal democratic values, and to prove he could change direction.
"Why should we believe that Britain will be better governed if he moves from No 11 Downing Street to No 10? Why should we believe that more of the same is what Britain needs?"
'Five tests'
Sir Menzies went on: "Britain needs a government that is prepared to reduce inequality and provide quality public services throughout the whole of Britain.
"To uphold the rule of law and to preserve our traditional freedoms, to take on the challenge of climate change and to restore Britain's international reputation.
"The question is - can Gordon Brown meet that challenge? Does he have the courage to take Britain in a new direction?"
He announced that he had devised five tests for the Chancellor.
He called on Mr Brown to "end Labour's authoritarian attack on civil liberties" by scrapping ID cards.
He urged Mr Brown to "grasp the challenge posed by climate change," and, thirdly, "break open the poverty trap".
His fourth test was to "trust the people" by devolving power to local people and the fifth was that "Britain's foreign policy should not be set in Washington".
If Mr Brown met these tests, "he will have changed direction and embraced liberal democracy," Sir Menzies told delegates.
He also stressed the party's commitment to electoral reform, saying the party's "ambition" was to have a "government elected by a system where every vote counts".
'Teenage kicks'
Sir Menzies, who received a four-minute standing ovation from delegates following his speech at the party's spring conference, coupled his call to Mr Brown with a scathing attack on Conservative leader David Cameron.
The Lib Dem leader, who has faced questions about his own age and whether, at 65 he is too old to lead his party, began by mocking recent revelations about Mr Cameron's youth.
"Come on Dave, it's time to come clean. Admit your guilty secret.
"In your youth you were a Tory Boy and your heroes were Michael Howard, Norman Lamont and John Selwyn Gummer.
"With pin-ups like that, frankly, I'd want to keep my past private too."
The Lib Dems have used their spring conference to move on to traditional Conservative territory on crime and taxation.
Sir Menzies accused Mr Cameron of "ducking and weaving" and questioned his judgement on his decision to support the Iraq war.
"Teenage kicks are one thing, but you've got to grow up sometime," he said.
